Considerable effort has been expended in recent years to develop high-reliability, low flow-rate, miniature pumps. Piezoelectric technology has played a significant part in this effort. The development in this area has lead to several pump designs where part of the pump assembly utilized the piezoelectric effect, while the balance of the design retains conventional approaches. For example, in U.S. Pat. No. 3,215,078 to Stec, a piezoelectric tube is used for an impellor, but not for valving. U.S. Pat. No. 3,963,380 to Thomas use piezoelectric impellors and magnetic valves. U.S. Pat. No. 3,107,630 to Johnson uses a piezoelectric impeller with spring-loaded valves. U.S. Pat. No. 3,150,592 to Stec comes closest to an all-piezoelectric design, but still requires mechanical check valves.
Hybrid piezoelectric-mechanical assemblies, such as those in the above-referenced prior art, suffer a number of significant limitations. As a primary consideration, the mechanical parts of these prior art assemblies add mass and friction to the design, thereby increasing both energy requirements and operational wear, as well as inversely reducing reliability. An additional drawback of such hybrid assemblies is the increased time factors associated with mass and friction. Increased time factors reduce efficiency as well as limit the availability of miniature control devices, such as integrated circuit microprocessors. Another drawback of hybrid pumps is the noise commonly associated with any mechanical device. Attempts to limit such noise usually contribute to inefficiency and increase the size of the overall device. Further, hybrid assemblies must have sufficient size to accommodate the mechanical components, and this factor contributes to the difficulty in miniaturizing such pumps.
Reliability of pumps, an ever increasing requirement, is normally accomplished by placing external sensors about the pump to determine if fluid flow has stopped or been substantially diminished. These external sensors are bulky and inconvenient to use. In addition, they are separate from the pump itself, thus requiring a need for moving the sensors every time the pump is moved.
Accordingly, a need has arisen for a pump wherein all moving parts are piezoelectrically motivated and, in addition, a pump providing the capability for readily monitoring the pumping operation.